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- To: <seaint(--nospam--at)seaint.org>
- Subject: eave blocking in high wind and seismic zones
- From: "Schwan, Martin K." <SchwanMK(--nospam--at)ci.anchorage.ak.us>
- Date: Wed, 5 May 2004 14:35:32 -0800
Title: eave blocking in high wind and seismic zones
This last winter in Anchorage, Alaska we observed an increase in ice damming on sloped roofs due to a seasonally large snow fall and recent construction practices. The snow on the roof melts and freezes on the unheated eaves. This can cause leaks by backing up melt water under the roof covering and causes unsightly icicles on the eaves. Although many buildings did not have problems, one suspected factor was insufficient ventilation at the eaves. Most residential construction locally is engineered and does not conform to the prescriptive conventional light frame construction provisions. It is current standard practice to require full height 2x blocks between truss heels to provide a lateral force load path from roof sheathing to exterior shear wall top plates. And we require a minimum 9” energy heel with boundary nailing into the blocking thru the roof diaphragm and normally we see A35’s specified for the blocking to shear wall top plate connection. The trusses/rafters are braced at the gable end and every 20’ thereafter. We are seismic design category D and wind speeds zones from 100 mph to 125 mph for 3-second gust (seismic zone 4 and wind speed 80-110 in UBC).
Several suggestions from the home builders include increasing the area of venting at the eave by eliminating the solid blocking required by code (see ’97 UBC 2320.12.8, 2000 IBC 2308.10.6). Note we have a handout to allow alternate half height blocking with 3-1 ½” holes in the full height blocking or full height blocking with 3-1 1/2” holes per truss bay. Our problem with eliminating the blocking all together is of course shear transfer and cross grain bending (CGB). They want us to apply common sense to the problem because there are lots of older houses without blocking that have withstood the test of time and of course “in the lower 48 nobody blocks at the eave.” Additionally they say the calculations do not consider repetitive members and one or two trusses may fail in CGB but a whole series with sheathing on them would not. I would appreciate your input.
1. Do you require eave blocking?
2. If the forces at the roof diaphragm were greater or less than about 200 plf would your answer to #1 change?
3. Are there any test results available with half height blocking in high wind areas and or high seismic zone?
4. Would flat or vertical blocking at the truss/rafter tail eliminate cross grain bending?
5. Do you see any problems with a discontinuous roof diaphragm at the roof peak because it was cut for ridge vent placement?
I thank you in advance for your time and comments…Martin
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